The field of the present invention is ultraviolet light-emitting “blacklight” lamps used in mineralogy, scientific research, and related fields.
Ultraviolet lamps are widely used by geologists, rockhounds, and collectors of minerals both for identification of mineral materials and for creating the spectacular displays of fluorescence that many minerals emit when illuminated with ultraviolet light. Some minerals, which may appear drab or colorless when viewed under normal visible light, upon irradiation with ultraviolet light emit bright colored visible light that is characteristic both of the mineral in question and of the wavelength of the ultraviolet light used in the illumination. For instance, one variety of the mineral calcite found in Mexico, normally of a whitish color, will fluoresce a bright blue-white color when illuminated with short-wave ultraviolet light, a yellow color when illuminated by longer wave lengths of ultraviolet light, and a bright pink color when illuminated with yet longer wave lengths of ultraviolet light.
Geologists, collectors and other devotees of mineralogy often use and greatly enjoy the spectacular visual displays produced by fluorescent minerals, using ultraviolet lamps of various types to produce this fluorescence. The ability to employ the various wavelengths of ultraviolet light, and view the resulting different effects on minerals, greatly adds to mineralogical knowledge and to the enjoyment of this field of study by professionals and amateurs alike.
Other fields of science also employ ultraviolet lamps for several purposes; for instance in the fields of molecular biology and biochemistry, ultraviolet light of various wavelengths is used to illuminate, on a transilluminator, electrophoresis gels which are used in the analysis of DNA and proteins. Ultraviolet lamps are also used in other applications such as forensics, where an ability to change readily between different ultraviolet and visible wavelengths of illumination, in a durable, less expensive apparatus, would also be desirable.
The ultraviolet light portion of the electromagnetic spectrum is normally divided into at least three portions, characterized by the wavelength of the light in question. Ultraviolet light is generally viewed as light having wavelengths less than about 400 nanometers (nm=10−9 m), which is about the shortest wavelength of light that the human eye can perceive as violet in color, and greater than about 200 nm. The limit of the ultraviolet portion of the electromagnetic spectrum at the short wavelength end is not absolutely fixed, but as ultraviolet light of wavelengths much shorter than 200 nm is strongly absorbed by the gases in air, light of wavelengths below 200 nm (usually called “far” or “vacuum” ultraviolet) is not typically used in applications contemplated for the present invention, especially as use of even shorter wavelengths of ultraviolet light acts to produce the toxic gas ozone from oxygen in air. Thus, as a practical matter, 200 nm is conveniently chosen as the short wavelength limit.
Within this range ultraviolet light is roughly divided into three sections: UV C or “short-wave” UV being of the shortest wavelengths from approximately 200 nm up to about 280 nm, UV B or “medium-wave UV” being of intermediate wavelengths from about 280 nm to about 320 nm, and UV A, or “long-wave” UV being of the longest wavelengths from about 320 nm up to 400 nm, where the visible light portion of the electromagnetic spectrum begins. These divisions are general in nature, with the dividing lines being somewhat indefinite. UV light sources of differing wavelengths have different effects, one of which is the color of visible light produced when some fluorescent materials or minerals are illuminated by various kinds of the UV light sources.
Ultraviolet light can be generated by a number of methods, but one of the commonest sources of ultraviolet light is the low pressure mercury vapor discharge lamp. In this type of lamp, electrical current passes through an essentially evacuated quartz or glass tube which contains small quantities of vaporized mercury metal plus small amounts of an inert “starter” gas. The effect of this electric current is to electronically excite the mercury atoms in such as way as to cause the emission of ultraviolet light of wavelengths characteristic for mercury vapor at low pressure, most notably ultraviolet light of wavelength 254 nm in the UV C wavelength band. U.S. Pat. No. 1,888,421 describes an apparatus of this type where mercury is added to evacuated electrical discharge tubes containing small amounts of various types of inert gases in order to produce the blue colored ultraviolet light emissions characteristic of mercury. This enables production of the characteristic emission wavelengths of low pressure mercury vapor in the UV C region of the ultraviolet light spectrum, but low pressure mercury vapor does not emit much ultraviolet light in the UV A or UV B wavelength bands.
Ultraviolet light in the UV A and UV B ranges is commonly produced through the use of phosphors, substances that emit light in the appropriate wavelength range of the ultraviolet spectrum when illuminated by UV light of shorter wavelengths. U.S. Pat. No. 2,135,732 discloses the use of luminescent (phosphorescent or fluorescent) materials coated on the inside of a mercury vapor discharge tube to produce light, of a different wavelength from that of the primary low pressure mercury vapor UV light emission. It should be noted that there are two terms employed in describing the emission of light from a material when it is illuminated by light of a different wavelength: fluorescence and phosphorescence. While both phenomena arise from very similar physical processes, they differ in the length of time that the light emission persists after the illuminating light is turned off, phosphorescence persisting longer than fluorescence. However, in the present context the terms may be considered to be interchangeable. Thus the terms phosphor and fluor have the same meaning in the present context, that is materials which emit light of a different wavelength or set of wavelengths from that with which they are illuminated to stimulate the emission.
Typically, ultraviolet light in the UV A and UV B spectral ranges is produced by coating the inside of the glass envelope of a UV C emitting lamp, usually of the low pressure mercury vapor discharge type, with various types of phosphors. When the electric current is passed through the low pressure mercury vapor, it emits its characteristic short-wave UV C light. This short-wave UV light illuminates the phosphor lining the inside wall of the glass envelope and causes it to emit light at its characteristic wavelength, UV A or UV B or even visible light, depending upon the identity of the phosphor. Thus, lamps of this type have only been able to produce a single type of ultraviolet illumination: UV C if there is no phosphor, or UV A or UV B or visible light if the glass envelope of the lamp is coated with a phosphor that characteristically emits light in one of these wavelength bands.
To build a lamp designed to selectably emit light in more than one band of UV light, a separate bulb has been used for each of the bands, UV A, UV B, or UV C. U.S. Pat. No. 5,387,801 provides an example. For UV C a low pressure mercury discharge bulb is used, and for UV B or UV A a low pressure mercury vapor discharge bulb lined with an appropriate type of phosphor is used. However, the need to use a different UV bulb for each wavelength band is both expensive and cumbersome, and requires turning each bulb on and off more frequently, which is well known to shorten bulb life.
There have been disclosures of lamps which have phosphors of more than one type within a single bulb; these bulbs have the advantage of being capable of producing light of more than a single wavelength band, yet in these lamps there has been no provision for selecting between one wavelength band and another at will. For instance U.S. Pat. No. 4,703,224 discloses coating the inside of a mercury vapor discharge tube with a mixture of two or more types of phosphors, some emitting UV A and some emitting UV B, in adjustable proportion set at the time of manufacture of the given bulb. While this allows the production of light in more than a single wavelength band of ultraviolet light, it fails to allow for selecting among different wavelength bands during the operation of the lamp.
Likewise U.S. Pat. No. 4,967,090 (“'090 patent”) and U.S. Pat. No. 5,557,112 (“'112 patent”) disclose coating the inside of mercury vapor discharge tubes with two or more different types of phosphor, each in a specific zone or sector on the inside of the cylindrical bulb. In the '090 patent, the two phosphor types are coated in different longitudinally-disposed areas, while in the '112 patent, one phosphor coats an area at one end of the interior of the cylindrical bulb and the other phosphor coats the rest of the interior of the bulb. These techniques allow production of more than a single wavelength band of light at a given time, but these devices are designed to give a fixed proportion of light in each of the different wavelength bands, to be set at the time of manufacture of the bulb, for the purposes of giving an optimal mixture of wavelength bands to facilitate effective suntanning.
U.S. Pat. No. 3,676,728 discloses a lamp capable of producing selectable illumination with one of a plurality of wavelength bands by using a plurality of types of phosphor coated on the interior of a mercury vapor discharge bulb, in which longitudinal stripes each consisting of one of two or more different phosphors are used. The desired wavelength band is selected by physical rotation of the bulb itself such that the phosphor emitting the desired wavelength is presented at the light exit port of the apparatus. However, this type of bulb suffers from the disadvantages of damage to phosphors and resulting relatively short phosphor lifetimes common to many phosphors that are exposed to the harsh conditions on the inside of a low pressure mercury vapor discharge bulb. This lamp also suffers from the further disadvantage that only a limited fraction of the total light energy emitted by the luminous mercury vapor within the bulb actually falls upon the phosphor selected. As the phosphors are situated within the bulb, it is unavoidable that much of the light energy will be wasted because there is no way to direct it toward the desired phosphor in exclusion of phosphors placed at other locations within the bulb.
It is Well known that phosphors coated on the interior wall of a mercury vapor discharge bulb suffer from exposure to the electric current, mercury atoms and ions, and short-wave ultraviolet light particularly of wavelengths less than 200 nm, for instance the 185 nm far UV emission of mercury vapor that can propagate within a vacuum but is normally stopped by most solid materials of UV bulb construction, and by air. U.S. Pat. No. 4,243,090 describes the loss of efficiency and drop in effective light production by phosphors due to these effects, and discloses an attempt to improve phosphor life within mercury vapor discharge bulbs through chemical treatment of the phosphors.
There have been attempts to reduce this type of damage and provide for longer phosphor life and corresponding reduced cost by placement of the phosphor outside the primary light source bulb. U.S. Pat. No. 5,736,744 discloses an apparatus for use as a transilluminator, a device for visualizing DNA bands in an electrophoresis gel, whereby light emitted from a primary ultraviolet light source may be converted to another wavelength band of UV or visible light by means of a phosphor external to the mercury vapor discharge bulb. An apparatus emitting light in the UV C range on which electrophoresis gels to be illuminated are placed is modified to emit other wavelengths of UV or visible light by placing a transparent plate coated with a phosphor over the light exit port of the apparatus. The phosphor emits light of a different wavelength range, such that the electrophoresis gels are illuminated by the newly-produced wavelength band of light, ultraviolet or visible. However, the system is cumbersome to use in that to shift from the wavelength band of the primary light source to light of a secondary wavelength band, a separate plate must be manually placed over the light exit port of the apparatus. The apparatus incorporates no easy or trouble-free way to shift between different emitted light wavelength bands other than by placing the phosphor-coated plate into a slot, or by swinging it into place via a hinge or pivot point mechanism. No provision is made for using more than a single phosphor-coated plate other than by manually removing one plate from a positioning slot and placing another in its place. Such a device would not be very useful when portability is important as is often the case, for instance, when one wishes to view mineral specimens in the field or do forensic studies at a crime scene. Such a device would also not be useful in applications where a user wished to use a non-manual method, such as an electric motor, to switch between various wavelength-transforming materials (phosphors).
Accordingly, there is a need for a simpler, less costly and more effective apparatus for producing various wavelengths of light, particularly ultraviolet light, from a primary light source.